The Seris Head stage (ion-selective). This component has been completely redesigned to overcome inherent limitations and noise. We have moved from the AD515 to the AD549 as the signal-input device (Analog Devices). The AD 549 has superior noise and input characteristics. We have also incorporated a low pass filter and a differential amplifier in the head stage to increase common mode rejection. The AD549 is socketed in a Teflon mounting with gold plated pins. This increases the operator's ability to easily repair the head stage. While resistors have been added to decrease the probability of the AD549 being damaged by static charges, we have found that failures still occur and easy replacement is necessary. The Serp Head stage (polarographic). The amperometric head stage used for the Seris-oxygen also uses the AD549 as its input device. However the AD549 is configured as a current to voltage converter (I to E) uses a resistor of 1000 MegOhms. This gives us an output of 1mV per pA. The polarization voltage is applied to the positive input of the AD549 to elevate the sensing node of the I to E converter to the polarization voltage. The output of the I to E converter is then sent to a differential amplifier along with the polarization voltage. The result is a voltage output proportional to the current, which in turn flows as a result of applying the polarization voltage in an oxygen gradient.